U.S. Energy Secretary Steven Chu announced today that he would be stepping down. Chu’s departure is not unexpected, and it’s not rare—most second Presidential administrations feature a lot of Cabinet turnover, and a number of Chu’s colleagues had already announced plans to leave. He’s said he’ll stick around until President Obama can find a successor, which might be several weeks.

Chu will be missed, as much—if not more—for who he was, as what he did. Chu’s record as Energy Secretary is mostly positive—as a Washington outsider, he was handed tends of billions of dollars in stimulus funding, and channeled it towards promising investments in clean tech that should continue to pay off in the years to come. He wasn’t perfect—some of those bets, like Solyndra, failed to pay off, and that Washington inexperience could hamper him. (Brad Plumer at the Washington Posthas a good scorecard on Chu’s four years.)

But Chu was the first person to ever join a Presidential Cabinet as a Nobel prize winner—and for physics, not one of those liberal arts Nobel prizes like peace. He was the embodiment of an ideal: that the truly best and the brightest could come to Washington to serve the public at our moment of need. Chu actually lived up to that Superscientist ideal during the 2010 Gulf of Mexico oil spill, when he helped lead the team that figured out a way to plug the blown out well. But Washington is Washington, and given the political microscope he’s been under for the past couple of years, I imagine Chu can’t be too sad to leave D.C. behind. At least he’ll having an easier time biking to work from now on.

Chu always knew that climate change was one of the biggest threats the U.S. and the world faced—see his pre-Secretary warnings on the danger of coal. He’ll leave office with U.S. carbon emissions at their lowest level since 1994, thanks to increased energy efficiency, more renewable energy and the switch from coal to natural gas. But he’ll also leave knowing that much more needs to be done. Chu ended his characteristically detailed final memo with a reminder of the ethical need to fight climate change:

Ultimately we have a moral responsibility to the most innocent victims of adverse climate change. Those who will suffer the most are the people who are the most innocent: the world’s poorest citizens and those yet to be born. There is an ancient Native American saying: “We do not inherit the land from our ancestors, we borrow it from our children.” A few short decades later, we don’t want our children to ask, “What were our parents thinking? Didn’t they care about us?”